Low-temperature impact-resistant polyamide-based stretch-oriented mutilayer film

ABSTRACT

A stretch-oriented multilayer film suited for use as a freeze packaging material, a deep drawing packaging material, a vertical pillow packaging material, etc., is provided as a stretch-oriented multilayer film, comprising at least three layers including a surface layer (a) comprising a thermoplastic resin, an intermediate layer (b) comprising a polyamide resin and a surface layer (c) comprising a sealable resin, said multilayer film exhibiting an impact energy of at least 1.5 Joule at a conversion thickness of 50 μm at −10° C. The multilayer film is produced through an inflation process using water having a large capacity as a cooling and a heating medium and including a combination of a high degree of stretching and a high degree of relaxation heat treatment not exercised heretofore.

TECHNICAL FIELD

[0001] The present invention relates to a stretch-oriented (i.e.,stretched and oriented) multilayer film including a polyamide resinlayer as a principal resin layer and having an excellent low-temperatureimpact resistance.

BACKGROUND ART

[0002] Hitherto, it has been widely practiced to use a stretch-orientedfilm for packaging processed livestock meat, fresh fish meat, raw meat,soup, etc. Particularly, for packaging of frozen food and vacuumpackaging livestock meat with bones and fish meat, such astretch-oriented packaging film is required to have excellentanti-pinhole property and mechanical strength, and a multilayer filmincluding a polyamide resin layer has been generally used therefor.

[0003] Particularly, in the case of requiring a relatively thinmultilayer film, a multilayer film including a stretched polyamide resinlayer has been suitably used.

[0004] For production of a multilayer film including a stretchedpolyamide resin layer, there has been generally practiced a processcomprising dry lamination or extrusion lamination of an unstretchedpolyolefin resin layer onto a stretched polyamide resin layer. However,in the case of necessitating excellent anti-pinhole property andmechanical strength, such a lamination film is caused to have acomplicated layer structure, which requires an increased number ofproduction steps and an increased film thickness for ensuring thestrength, thus leading to an increased production cost.

[0005] For the purpose of reducing the production steps, the multilayerstretching of a co-extruded multilayer film has been commercialized, butthis is accompanied with a problem such that the produced film is liableto have an excessively large shrinkability, which makes difficult thesecondary processing, such as bag-making, printing or packaging by meansof an automatic packaging machine. Particularly, in the case of packinga soup in a hot state into a pouch of such a multilayer film or byautomatic packaging with such a multilayer film, the packing becomesdifficult due to the film shrinkage.

[0006] It has been known that a multilayer film including a stretchedpolyamide resin layer has an excellent low-temperature strength, but ithas been also believed inevitable that the strength is remarkablylowered at a temperature as low as −10° C. for packaging a frozenproduct.

DISCLOSURE OF INVENTION

[0007] A principal object of the present invention is to provide apolyamide-based stretch-oriented multilayer film having an improvedlow-temperature impact resistance.

[0008] Another object of the present invention is to provide apolyamide-based stretch-oriented multilayer film excellent in mechanicalproperties, such as piercing strength and anti-pinhole property.

[0009] A further object of the present invention is to provide apolyamide-based stretch-oriented multilayer film which is free fromexcessive shrinkability and is excellent in boiling resistance,processability as by deep drawing or adaptability to packaging machines.

[0010] According to our study, it has become possible to obtain apolyamide-based stretch-oriented multilayer film having a remarkablyimproved low-temperature impact resistance while retaining ordinaryphysical properties such as a tensile strength by duly noting theposition of the polyamide resin layer and effecting appropriatestretching process and post-heat treatment. It has been also discoveredthat the multilayer film is also provided with remarkable improvement inthe above-mentioned objective physical properties and processingcharacteristics, which are presumably attributable to a molecularorientation state of the film having provided the improvement inlow-temperature impact resistance.

[0011] According to the present invention based on the above knowledge,there is provided a stretch-oriented multilayer film, comprising atleast three layers including a surface layer (a) comprising athermoplastic resin, an intermediate layer (b) comprising a polyamideresin and a surface layer (c) comprising a sealable resin, saidmultilayer film exhibiting an impact energy of at least 1.5 Joule at aconversion thickness of 50 μm at −10° C.

[0012] The present invention further provides a preferred process forproducing the above-mentioned stretch-oriented multilayer film. Thus,according to the present invention, there is also provided a process forproducing a stretch-oriented multilayer film, comprising the steps of:

[0013] co-extruding at least three species of melted thermoplasticresins to form a tubular product comprising at least three layersincluding an outer surface layer (a) comprising a thermoplastic resinother than polyamide resin, an intermediate layer (b) comprising apolyamide resin and an inner surface layer (c) comprising a sealableresin,

[0014] cooling with water the tubular product to a temperature below alowest one of the melting points of the thermoplastic resin, thepolyamide resin and the sealable resin constituting the layers (a), (b)and (c),

[0015] re-heating the tubular product to a temperature which is at mostthe lowest one of the melting points of the thermoplastic resin, thepolyamide resin and the sealable resin constituting the layers (a), (b)and (c),

[0016] vertically pulling the tubular product while introducing a fluidinto the tubular product to stretch the tubular product in the verticaldirection and the circumferential direction, thereby providing abiaxially stretched tubular film,

[0017] folding the tubular film,

[0018] again introducing a fluid into the folded tubular film to form atubular film,

[0019] heat-treating the tubular film from its outer surface layer (a)with steam or warm water until a relaxation ratio reaches at least 20%in at least one of the vertical direction and the circumferentialdirection, and

[0020] cooling the heat-treated tubular film to provide astretch-oriented multilayer film exhibiting an impact energy of at least1.5 Joule at a conversion thickness of 50 μm at −10° C.

[0021] Some history and details as to how we have arrived at the presentinvention as a result of study for achieving the above object, will nowbe briefly discussed.

[0022] As mentioned above, it has been well known that a multilayer filminducing a polyamide resin layer as a principal layer provides apackaging material excellent in various properties. Our research grouphas also disclosed a stretch-oriented multilayer film having a basiclaminate structure of polyester resin/polyamide resin/sealable resin asa heat-shrinkable film with excellent physical properties (JP-A 4-99621,U.S. Pat. No. 5,336,549), and has further disclosed that a multilayerfilm having such a laminate structure can achieve an effective reductionin heat-shrinkage stress which can be obstacle to automatic packaging,while retaining a preferred degree of heat-shrinkability (JP-A11-300914, WO 99/55528). Particularly, the latter WO '528 publicationdiscloses a process including steps of subjecting a biaxially stretchedtubular film of the above-mentioned laminate structure after stretchingat a ratio of 2.5-4.0 times both in a vertical direction and in acircumferential direction to a heat treatment with steam or warm waterat 60-98° C. and then cooling the heat-treated film, to proved abiaxially stretched film exhibiting a heat-shrinkage stress at 50° C. ofat most 3 MPa both in longitudinal direction and in transversedirection, and a hot water shrinkability at 90° C. of at least 20%. Theinvention of the WO '528 publication aims at production of aheat-shrinkable film, and in its Examples, the relaxation ratio in thepost-heat treatment after the biaxial stretching remained at a level of15% at the most. In contrast thereto, we have tried a heat treatmentwith steam or warm water causing a larger relaxation ratio of 20% orhigher after a biaxial stretching at ratios identical to or even higherthan those adopted in the WO '528 publication. As a result thereof, ithas been unexpectedly found possible to obtain a remarkably improvedlow-temperature impact resistance represented by an impact energy at−10° C. while retaining ordinary film strength as represented by atensile strength.

[0023] As an ordinary practice, a highly stretched film is not daringlysubjected to heat treatment for a high degree of relaxation. A firstreason thereof is that a heat treatment for a high degree of relaxationhas not been believed desirable since such a relaxation heat treatmentfunctions to deliberately lower a rigidity and a strength of a film, theincrease of which is a principal purpose of a high degree of stretching.More specifically, a relaxation heat treatment of a highly stretched,even if performed, should be suppressed to such a level as to moderate adifficulty accompanying a high degree of stretching, i.e., anexcessively large heat shrinkage stress, and a higher degree ofrelaxation heat treatment, if performed, has been believed to onlyresult in a decrease of the effect of the high degree of stretching andnot provide an additional effect. As another reason, one advantageattained by a high degree of stretching is the possibility of obtaininga film of a large width by using a film forming apparatus of arelatively small scale. However, a high degree of relaxation heattreatment functions to completely negate the effect, and requires alarge scale apparatus for producing a film of a large width. Further, ahigh degree of relaxation heat treatment lowers the productivity of filmon an area-basis and can result in a remarkably lower yield due tooccurrence of film products failing to satisfy a regulation of width.

[0024] Unexpectedly, however, it has been found possible to obtain aremarkable increase in low-temperature impact resistance by the processof the present invention represented by the above-mentioned combinationof high degree of stretching and high degree of relaxation heattreatment. The reason thereof has not been fully clarified as yet butmay be presumed as follows. The film of the present invention at a stageafter a high degree of stretching (that is inevitably required forallowing a subsequent high degree of relaxation heat treatment since adegree of relaxation exceeding a hot water shrinkability of a film afterthe stretching treatment is impossible) is composed of film-constitutingmolecules including a crystalline portion and an amorphous portion whichhave been both molecular-oriented to provide increased tensile strength,etc. The film in this state is liable to show a reduced elongation atbreakage, but the orientation of the amorphous portion is sufficientlyrelaxed owing to a subsequent high degree of relaxation heat treatmentwhile retaining the orientation of the crystalline portion, whereby aremarkable increase in elongation at breakage is understood to resultwhile retaining absolute strengths such as a tensile strength. Further,the increased elongation at breakage due to the relaxation of theamorphous portion is understood to be associated with improved otherfilm properties also aimed at by the present invention, i.e.,improvements in piercing strength, anti-pinhole property, deep-drawingprocessability requiring easiness of elongation, vertical pillowpackaging (or vertical type forming, filing and closing packaging)characteristic requiring a thin, pliable but tough film,lid-adaptability and boiling resistance requiring low shrinkability,etc.

[0025] As described above, in order to obtain a stretch-orientedmultilayer film having an improved low-temperature impact resistanceaccording to the present invention, a combination of a high degree ofstretching and a high degree of relaxation heat treatment is essential.For realizing the combination, it is essential to include a principalresin layer comprising a polyamide resin which is adapted to a highdegree of stretching and acquires remarkably improved mechanicalstrength thereby. In the process of the present invention, the polyamideresin layer is further used as an intermediate layer to realize the highdegree of stretching—high degree of relaxation heat treatment. Morespecifically, in order to allow the high degree of stretching—highdegree of relaxation heat treatment, the resinous tubular product(parison) after the heat melting extrusion is quenched with water as acooling medium exhibiting good heat efficiency, whereby a stretchingstress is effectively applied to resin molecules in the subsequentbiaxial stretching step. Further, also in the subsequent post-heattreatment, a high degree of relaxation heat treatment is performedeffectively by using steam or warm water as a heating medium having alarge heat capacity. However, the polyamide resin ismoisture-absorptive, so that if the polyamide resin is exposed to thesurface layer, the resinous tubular product after the melt extrusionabsorbs water at the time of water quenching to lower the effect of thehigh degree of stretching treatment. Accordingly, in the process of thepresent invention, a polyamide resin layer excellent in stretchabilityand mechanical properties after the stretching is used as a principalintermediate layer, and water showing excellent thermal efficiency asheating and cooling media to realize the high degree of stretching—highdegree of relaxation heat treatment, thereby succeeding in production ofa stretch-oriented multilayer film having a remarkably increasedlow-temperature impact resistance.

BRIEF DESCRIPTION OF THE DRAWING

[0026]FIG. 1 the sole figure in the drawing, is a schematic illustrationof an apparatus system suitable for practicing an embodiment of theprocess for producing a stretch-oriented multilayer film according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The stretch-oriented multilayer film according to the presentinvention comprises at least three layers including a surface layer (a)comprising a thermoplastic resin, an intermediate layer (b) comprising apolyamide resin and a surface layer (c) comprising a sealable resin.

[0028] The thermoplastic resin constituting the surface layer (a) isrequired to provide a surface layer (a) which, in the state oflamination with the intermediate layer (b) comprising a polyamide resin,is required to exhibit an appropriate degree of stretchability andobstruct moisture penetration to the intermediate layer. Thethermoplastic resin may preferably have a lower moisture-absorptivitythan the polyamide resin. Preferred examples of the thermoplastic resinmay include: polyolefin resins which have been conventionally used forpolyamide resin-based laminate films, inclusive of polyethylenes, suchas LLDPE (linear low-density polyethylene), VLDPE (linear verylow-density polyethylene) and LDPE (low-density polyethylene)(polyethylenes herein including those polymerized with single-sitecatalysts (or metallocene catalysts) in addition to those polymerized byconventional catalysts (Ziegler-Natta catalysts)); polypropylene,propylene-ethylene copolymer, propylene-ethylene-butene-1 copolymer,ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer,ethylene-methacrylic acid copolymer, and ethylene-ethyl acrylatecopolymer, wherein comonomers other than olefin occupy a relativelyminor proportion (below 50 wt. %); and also polyester resin, etc. Amongthese, a polyester resin is excellent in surface properties, such astransparency, surface hardness, printability and heat resistance, and isa particularly preferable material for the surface layer (a).

[0029] The polyester resin (PET) preferably constituting the surfacelayer (a) may comprise either an aliphatic polyester resin or anaromatic polyester resin.

[0030] More specifically, examples of dicarboxylic acids constitutingthe polyester resin may include: terephthalic acid, isophthalic acid,phthalic acid, 5-t-butylisophthalic acid, naphthalenedicarboxylic acid,diphenyl ether dicarboxylic acid, cyclohexane-dicarboxylic acid, adipicacid, oxalic acid, malonic acid, succinic acid, agelaic acid, sebacicacid, and dimer acids comprising dimers of unsaturated fatty acids.These acids may be used singly or in combination of two or more species.Examples of diols constituting the polyester resin may include: ethyleneglycol, propylene glycol, tetramethylene glycol, neopentyl glycol,hexamethylene glycol, diethylene glycol, polyalkylene glycol,1,4-cyclohexane-dimethanol, 1,4-butanediol, and 2-alkyl-1,3-propanediol. These diols may be used singly or in combination of two or morespecies.

[0031] Among these, it is preferred to use an aromatic polyester resinincluding an aromatic dicarboxylic acid component, particularlypreferably a polyester formed from terephthalic acid as the dicarboxylicacid and a diol having at most 10 carbon atoms, such as polyethyleneterephthalate or polybutylene terephthalate. It is also preferred to usea co-polyester resin formed by replacing a portion, preferably at most30 mol. %, more preferably at most 15 mol. %, of the terephthalic acidwith another dicarboxylic acid, such as isophthalic acid, or acopolyester resin between terephthalic acid and a mixture of diols, suchas ethylene glycol and 1,4-cyclohexanediol (e.g., “Kodapack PET#9921”,available from Eastoman Kodak Co.).

[0032] The polyester resin may preferably be one having an intrinsicviscosity of ca. 0.6-1.2. The outer surface layer (a) can contain up to20 wt. % of a thermoplastic resin other than the polyester resin, suchas a thermoplastic elastomer as represented by thermoplasticpolyurethane, or a polyolefin resin modified with an acid, such asmaleic acid, or an anhydride thereof.

[0033] The thickness of the surface layer (a) comprising a thermoplasticresin other than polyamide resin may preferably be smaller than that ofthe intermediate layer (a), particularly at least 6% and below 50% ofthat of the intermediate layer (b), so as not to impair the excellentstretchability and mechanical properties of the intermediate layer (b)comprising a polyamide resin.

[0034] Examples of the polyamide resin (PA) constituting theintermediate layer (b) may include: aliphatic polyamides, such as nylon6, nylon 66, nylon 11, nylon 12, nylon 69, nylon 610 and nylon 612; andaliphatic co-polyamides, such as nylon 6/66, nylon 6/69, nylon 6/610,nylon 66/610, and nylon 6/12. Among these, nylon 6/66 and nylon 6/12 areparticularly preferred in view of moldability and processability. Thesealiphatic (co-)polyamides may be used singly or in mixture of two ormore species. It is also possible to use a blend of such an aliphatic(co-)polyamide with a minor amount of an aromatic polyamide. Herein, thearomatic polyamide means a polycondensation product between a diamineand a dicarboxylic acid, at least one of which contains at leastpartially an aromatic unit. An aromatic co-polyamide is preferred.Examples thereof may include: a copolymer of an aliphatic nylon and anaromatic polyamide including an aromatic diamine unit, such as nylon66/610/MXD6 (wherein “MXD6” represents polymetaxylylene adipamide), anda copolymer of an aliphatic nylon and an aromatic polyamide including anaromatic carboxylic acid unit, such as nylon 66/69/6I, nylon 6/6I andnylon 6I/6T (wherein “(nylon) 6I” represents polyhexamethyleneisophthalamide, and “(nylon) 6T” represents polyhexamethyleneterephthalamide). These polyamide resins may be used singly or inmixture so as to provide a melting point of preferably 160-210° C. Theintermediate layer (b) can contain up to ca. 30 wt. % of a thermoplasticresin other than the polyamide resin, such as a polyolefin resinmodified with an acid, such as maleic acid, or an anhydride thereof,ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer,ionomer resin, or (partially) saponified ethylene-vinyl acetatecopolymer.

[0035] The sealable resin constituting the inner surface layer (c) maybe appropriately selected from thermoplastic resins inclusive of:polyolefins polymerized by using a single-site catalyst or metallocenecatalyst (sometimes abbreviated as “SSC”) inclusive of linearlow-density polyethylene (abbreviated as “SSC-LLDPE”) and verylow-density polyethylene (abbreviated as “SSC-VLDPE”); conventionaltypes of ethylene-α-olefin copolymers inclusive of “LLDPE” and “VLDPE”in terms of generally accepted abbreviations; ethylene-vinyl acetatecopolymer (abbreviated as “EVA”), ethylene-methacrylic acid copolymer(abbreviated as “EMAA”), ethylene-methacrylic acid-unsaturated aliphaticcarboxylic acid copolymer, low-density polyethylene, ionomer resin(abbreviated as “IO (resin)”), ethylene-acrylic acid copolymer,ethylene-methyl acrylate copolymer (abbreviated as “EMA”), andethylene-butyl acrylate copolymer (abbreviated “EBA”). Such a preferredclass of sealable resins may be termed as an ethylene copolymer,typically a copolymer of a major amount (i.e., more than 50 wt. %) ofethylene with a minor amount (i.e., less than 50 wt. %, preferably up to30 wt. %) of a vinyl monomer copolymerizable with ethylene selected fromthe group consisting of α-olefins having 3 to 8 carbon atoms, andunsaturated carboxylic acids and unsaturated esters of carboxylic acidshaving up to 8 carbon atoms, inclusive of acrylic acid, methacrylicacid, acrylate esters, methacrylate esters and vinyl acetate, or anacid-modified product of the ethylene copolymer (preferably modifiedwith up to 3 wt. % of an unsaturated carboxylic acid). It is alsopossible to use a thermoplastic resin, such as thermoplastic resin, suchas polypropylene resin, polyester resin or aliphatic nylon. The sealableresin may preferably have a melting point of at most 150° C., morepreferably at most 135° C. It is also possible to use a blend includingat least one species of such a sealable resin within an extent of notimpairing the transparency of the resultant film or a sealed productthereof.

[0036] Among the above, preferred examples of such sealable resinsconstituting the inner surface layer (c) may include: SSC-LLDPE,SSC-VLDPE, LLDPE, VLDPE, EVA, EMAA, ethylene-methacrylicacid-unsaturated aliphatic carboxylic acid copolymer, and IO resins. Aparticularly preferred class of SSC-type polyolefins may include thoseobtained by using a constrained geometry catalyst (a type of metallocenecatalyst developed by Dow Chemical Company). The constrained geometrycatalyst may provide ethylene-α-olefin copolymers which may beclassified as a substantially linear polyethylene resin having ca.0.01-ca. 3, preferably ca. 0.01-ca. 1, more preferably ca. 0.05-ca. 1,long-chain branching(s) per 1000 carbon atoms. Because of long-chainbranches each having ca. 6 or more carbon atoms selectively introducedinto its molecular structure, the ethylene-α-olefin copolymer may beprovided with excellent physical properties and good formability orprocessability, and an example thereof is commercially available fromDow Chemical Company under a trade name of “AFFINITY” or “ELITE”(including 1-octene as α-olefin).

[0037] Other examples of polyethylene resins obtained by using ametallocene catalyst may include those available under trade names of“EXACT” (EXXON Co.), “UMERIT” (Ube Kosan K.K.), “EVOLUE” (Mitsui KagakuK.K.), “COLONEL” (Nippon Polychem K.K.) and “HARMOLEX” (NipponPolyolefin K.K.).

[0038] Such a metallocene-catalyzed polyolefin (SSC-polyolefin) maypreferably have a polydispersity index defined as a ratio (Mw/Mn)between a weight-average molecular weight (Mw) and a number-averagemolecular weight (Mn) of below 3, more preferably 1.9-2.2.

[0039] The surface layer (c) comprising a sealable resin may preferablyhave a heat resistance which is lower than that of the surface layer(a). This is because, if the surface layer (c) comprising a sealableresin has a higher heat resistance than the surface layer (a), at thetime of applying heat to the film for sealing or deep drawing, thesurface layer (a) is liable to be melted in contact with a heating plateto result in a problem regarding the adaptability to sealing orpackaging machine or deep drawing processability.

[0040] The surface layer (c) comprising a sealable resin can be providedwith easy peelability, e.g., in the case of deep drawing packaging. Thiscan be accomplished by using, e.g., a mixture of EMAA and apolypropylene resin, or a mixture of EVA and polypropylene resin.

[0041] The heat-shrinkable multilayer film according to the presentinvention includes the above-mentioned surface layer (a) comprising athermoplastic resin, intermediate layer (b) comprising a polyamideresin, and surface layer (c) comprising a sealable resin, as itsindispensable component layers, but can also include an additionalintermediate layer other than the intermediate layer (b) comprising apolyamide resin for the purpose of, e.g., providing the productmultilayer film with improved functionality or processability. Examplesof such an optional intermediate layer may include the following.

[0042] A gas barrier intermediate layer (d), particularly an-oxygengas-barrier layer, comprising a gas barrier resin, examples of which mayinclude: EVOH; aromatic polyamides including an aromatic diamine unit,such as polymethacrylene adipamide (nylon MXD6); and amorphous aromaticpolyamides including an aromatic carboxylic acid unit, such aspolyhexamethylene isophthalamide/terephthalamide (nylon 6I/6T) which isa copolymer of isophthalic acid, terephthalic acid andhexamethylenediamine.

[0043] Another type of preferable intermediate layer may comprise acopolymer of ethylene and at least one species of monomer containing anoxygen atom in its molecule. Specific examples thereof may include: EVA,EMAA, ethylene-methacrylic acid-unsaturated aliphatic carboxylic acidcopolymer, EMA, EAA, EBA and IO resin.

[0044] Further, a layer of metallocene-catalyzed polyolefin having adensity below 0.900 g/cm₃ exhibits a good stretch orientationcharacteristic and may preferably be inserted as an optionalintermediate layer for providing a multilayer film having a largeheat-shrinkability at a stage after the biaxial stretching.

[0045] One or more adhesive resin layers may be inserted as an optionalintermediate layer, as desired, e.g., in case where a sufficientadhesion is not ensured between the above-mentioned respective layers.Such an adhesive resin can be selected from those constituting theabove-mentioned optional intermediate layers. Further preferred examplesof the adhesive resin used for the above purpose may include: EVA, EEA,EAA, acid-modified polyolefins (inclusive of reaction products betweenolefin homo-or co-polymers and unsaturated carboxylic acids, such asmaleic acid and fumaric acid, acid anhydrides, esters or metal salts ofthese acids, such as acid-modified VLDPE, acid modified LLDPE andacid-modified EVA). It is particularly suitable to use a polyolefinresin modified with an acid such as maleic acid or an anhydride thereof.

[0046] Into any one or more of the above-mentioned layers, it ispossible to add an additive, such as a lubricant or an antistatic agent.

[0047] Examples of the lubricant may include: hydrocarbon lubricants,fatty acid lubricants, fatty acid amide lubricants, ester lubricants andmetallic soaps. The lubricants may be liquid or solid. Specific examplesof the hydrocarbon lubricants may include: liquid paraffin, naturalparaffin, polyethylene wax and micro-crystalline wax. Fatty acidlubricants may include stearic acid and lauric acid. Fatty acid amidelubricants may include: stearic acid amide, palmitic acid amide,N-oleyl-palmitic acid amide, behenic acid amide, erucic acid amide,arachidic acid amide, oleic acid amide, methylene-bis-stearoyl amide,and ethylene-bis-stearoyl amide. Ester lubricants may include butylstearate, hardened castor oil, ethylene glycol monostearate, and stearicacid mono-glyceride. Metallic soaps may be derived from fatty acidshaving 12-30 carbon atoms and may include zinc stearate and calciumstearate as representative examples. Among these, fatty acid amidelubricants and metallic soaps may be preferred because of goodcompatibility with a thermoplastic resin, particularly a polyolefinicresin. Specifically preferred examples of lubricants may include behenicacid amide, oleic acid amide and erucic acid amide. These lubricants maypreferably be added in the form of a master batch. Such a master batchcontaining, e.g., 5-20 wt. % of a lubricant, may preferably be added inan amount sufficient to provide a concentration of 0.05-2 wt. % of thelubricant in a resin layer concerned.

[0048] The antistatic agent may preferably be a surfactant, which may beany of anionic surfactants, cationic surfactants, nonionic surfactants,amphoteric surfactants and mixtures of these. The anti-static agent maypreferably be added in a proportion of 0.05-2 wt. %, more preferably0.1-1 wt. % of a resin layer to which it is added.

[0049] Preferred examples of layer structure of the stretch-orientedmultilayer film according to the present invention are shown below.These are however not exhaustive.

[0050] (1) polyester resin/adhesive resin/polyamide resin/ adhesiveresin/sealable resin,

[0051] (2) polyester resin/adhesive resin/polyamide resin/ gas barrierresin/adhesive resin/sealable resin,

[0052] (3) polyester resin/adhesive resin/polyamide resin/ adhesiveresin/gas barrier resin/adhesive resin/ sealable resin,

[0053] (4) polyester resin/adhesive resin/polyamide resin/ adhesiveresin/gas barrier resin/adhesive resin/ polyamide resin/adhesiveresin/sealable resin,

[0054] (5) polyester resin/adhesive resin/polyamide resin/ gas barrierresin/polyamide resin/adhesive resin/ sealable resin,

[0055] (6) polyolefin resin/adhesive resin/polyamide resin/ adhesiveresin/sealable resin,

[0056] (7) polyolefin resin/adhesive resin/polyamide resin/ gas barrierresin/adhesive resin/sealable resin,

[0057] (8) polyolefin resin/adhesive resin/polyamide resin/ adhesiveresin/gas barrier resin/adhesive resin/ sealable resin,

[0058] (9) polyolefin resin/adhesive resin/polyamide resin/ adhesiveresin/gas barrier resin/adhesive resin/ polyamide resin/adhesiveresin/sealable resin, and

[0059] (10) polyolefin resin/adhesive resin/polyamide resin/ gas barrierresin/polyamide resin/adhesive resin/ sealable resin.

[0060] The stretch-oriented multilayer film may preferably be formed bylaminating the above-mentioned layers, followed by stretching andrelaxation into a final form of multilayer film having a total thicknessof 20-250 μm, particularly 40-150 μm. Further, in the case of packagingrib (meat) retaining a sharp-cut bone and requiring an especially highpinhole resistance, the total thickness may preferably be 60-250 μm,particularly 90-150 μm.

[0061] More specifically, it is preferred that the surface layer (a)comprising a thermoplastic resin has a thickness of 0.5-25 μm,particularly 1-15 μm, the intermediate layer (b) comprising a polyamideresin has a thickness of 3-50 μm, particularly 10-40 μm, and the surfacelayer (c) comprising a sealable resin has a thickness of 10-150 μm,particularly 15-60 μm. Particularly, in the case where the surface layer(a) comprises a polyester resin, it is preferred that the layer (a) hasa thickness smaller than that of the layer (b), more specifically athickness of 3-70%, particularly 6-50%, of that of the layer (b), inorder to provide the multilayer film with a properly harmonized biaxialstretchability.

[0062] The optionally disposed gas barrier layer (d) may have athickness of, e.g., 1-30 μm, preferably 2-15 μm. Below 1 μm, the oxygengas barrier-improving effect may be scarce, and above 30 μm, theextrusion of the layer and the stretching and processing of themultilayer film become difficult.

[0063] The adhesive resin layer can be disposed in a plurality oflayers, each having a thickness in the range of suitably 0.5-5 μm.

[0064] The stretch-oriented multilayer film may be formed by firstforming a yet-unstretched film by co-extrusion through a plurality ofextruders and then biaxially stretching the film by a known process,such as the tenter process, followed by a high degree of relaxation heattreatment at a relaxation ratio of at least 20% in at least one axialdirection. The stretching ratio may preferably be 2.5-4 times in bothlongitudinal and transverse directions. The thus-formed stretch-orientedmultilayer film can also be laminated with another resin layer accordingto a known lamination process.

[0065] The stretch-oriented multilayer film may preferably be formedthrough inflation according to the process of the present invention. Apreferred embodiment thereof is described with reference to FIG. 1, thesole figure in the drawing.

[0066] A number of extruders 1 (only one being shown) are providedcorresponding to the number of laminated resin species, and therespective resins from the extruders are co-extruded through an annulardie 2 to form a tubular product (parison) 3 including at least threelayers of an outer surface layer (a) comprising a thermoplastic resin,an intermediate layer (b) comprising a polyamide resin and an innersurface layer (c) comprising a sealable resin. The parison 3 is thenvertically pulled down into a water bath 4 and taken up by pinch rollers5 while being cooled down to a temperature that is below the lowest oneof the melting points of the principal resins constituting therespective resin layers (i.e., the thermoplastic resin, the polyamideresin and the sealable resin), preferably to 40° C. or below. Thethus-taken-up tubular film 3 a, while optionally introducing an openingaid such as soybean oil thereinto as desired, is introduced into a bath6 of warm water at, e.g., 80-95° C., which is at most the lowest one ofthe meting points of the principal resins constituting the respectivelayers, and the thus-warmed tubular film 3 b is pulled upwards to form abubble of tubular film 3C with fluid air introduced between pairs ofpinch rollers 7 and 8, whereby the tubular film 3C is biaxiallystretched simultaneously at a ratio of preferably 2.5-4 times, morepreferably 2.8-4 times, in each of vertical or machine direction (MD)and transverse or lateral direction (TD), most preferably at 2.9-3.5times (MD) and 3-3.5 times (TD), while cooling the film 3C with cool airat 10-20° C. from a cooling air ring 9. The thus biaxially stretchedfilm 3 d is once folded or laid flat and then pulled downwards to againform a bubble of tubular film 3 e with fluid air introduced betweenpairs of pinch rollers 10 and 11. The bubble of tubular film 3 e is heldwithin a heat-treating tube 12 wherein steam from blowing ports 13 isblown (or warm water from spraying ports is sprayed) against the tubularfilm 3 e to heat-treat the tubular film 3 e after the biaxial stretchingat 70-98° C., preferably 75-95° C., for ca. 1-20 sec., preferably ca.1.5 -10 sec., thereby allowing the tubular film to relax by 15-40% (butat least 20% in at least one direction), preferably 20-35%, in each ofthe machine direction (MD) and the transverse direction (TD). A tubularfilm 3 f after the heat-treatment corresponds to a stretch-orientedmultilayer film according to the present invention and is wound about atake-up or winding roller 14.

[0067] Again to say, in order to realize improvements in variousproperties represented by an improved low-temperature impact resistancewhile retaining excellent strengths, it is extremely preferred to adopta combination of high degree of stretching and high degree of relaxationtreatment, i.e., to ensure high stretching ratios of 2.5-4 times, morepreferably 2.8-3.5 times, in both MD/TD, most preferably 2.9-3.5 times,in MD and 3-3.5 times in TD and then to effect a heat-treatment forcausing relaxation by 15-40% in each of MD/TD (but at least 20% in atleast one direction), preferably by 20-30% in each of MD/TD, with steamor warm water as a heating medium having a large heat capacity. At alower stretching ratio, it is difficult to attain necessary filmstrengths after the heat treatment, and the resultant film is liable tohave thickness irregularity, thus failing to exhibit satisfactorypackaging performance. On the other hand, in the case of using a heatingmedium having a small heat capacity, such as hot air, or adopting alower heat treatment temperature of below 70° C., it becomes difficultto realize a sufficiently large degree of relaxation, thus being liableto fail in realizing a necessary improvement in low-temperature impactresistance. On the contrary, if the heat treatment is effected at ahigher temperature exceeding 100° C., the sealable resin layer (c)comprising, e.g., a polyolefin, is liable to be melted, whereby theorientation of the layer (c) is removed, thus being liable to fail inproviding excellent strength. If the relaxation percentage is below 15%at the time of the heat treatment, it is difficult to realize asufficient degree of orientation relaxation at the amorphous portion asrepresented by a desired low-temperature impact resistance. Above 40%,the resultant film is liable to be wrinkled.

[0068] The thus-obtained stretch-oriented multilayer film according tothe present invention retains a high degree of basic strengthrepresented by a tensile strength as a result of the high degree ofstretching of the polyamide resin layer and is also provided with aremarkably improved low-temperature impact resistance represented by animpact energy at −10° C. Further, accompanying the increase inlow-temperature impact resistance, the film has been also provided withremarkable improvements in piercing strength, anti-pinhole property,deep drawing characteristic, etc. Through the high degree of relaxationheat treatment, the heat-shrinkability of the product multilayer film isnaturally lowered. Thus, the stretch-oriented multilayer film of thepresent invention does not include a heat-shrinkability as an essentialproperty but may preferably retain a certain degree of hot-watershrinkability depending on the use thereof since such a degree of hotwater shrinkability provides an improved appearance by preventing theoccurrences of winkles of a packaged product.

[0069] Examples of appropriate degrees of hot-water shrinkability (at90° C.) for specific packaging materials may include: 0-20%, morepreferably 0 -15%, for freeze packaging material; 0-25%, more preferably0-15%, for deep drawing packaging material; 5-20%, more preferably5-15%, for tray packaging lid material; and below 15%, more preferablybelow 10% (below 15%, more preferably below 10% in terms of dryheat-shrinkability at 120° C.) for vertical pillow packaging material.Such a level of hot-water shrinkability of a product stretch-orientedmultilayer film can be controlled within an extent of retainingnecessary low-temperature impact resistance by adjusting the relaxationpercentage (within an extent of ensuring at least 20% in at leas onedirection) in connection with the preceding stretching ratio.

[0070] In order to provide a freeze-packaged product or a packagedproduct for cold circulation around 5° C. (or 0-10° C.) with an improvedpinhole resistance, the stretch-oriented multilayer film of the presentinvention may preferably show an actual impact resistance (i.e., not anormalized impact resistance at a thickness of 50 μm) of at least 1.6Joule at −10° C.

[0071] Particularly in the case of packaging of rib (meat) retaining asharp-cut bone and requiring an especially high pinhole resistance, themultilayer film may preferably show an actual impact resistance (asmeasured at −10° C.) of at least 3 Joule, more preferably at least 4Joule, further preferably at least 5 Joule, so as to allow the packagingwithout using a bone guard (reinforcing material) ordinarily used forsuch packaging.

[0072] In the above-described stretch-oriented multilayer filmproduction process according to the present invention, the multilayerfilm before or after the stretching may be exposed to radiation. By theexposure to radiation, the product multilayer film may be provided withimproved heat resistance and mechanical strength. Because of a moderatecrosslinking effect thereof, the exposure to radiation can exhibit aneffect of providing improved film formability by stretching and improvedheat resistance. In the present invention, known radiation, such as αrays, β rays, electron beams, Γ rays, or X rays may be used. In order toprovide an adequate level of crosslinking effect, electron rays and Γrays are preferred, and electron beams are particularly preferred inview of facility of handling and high processing capacity in producingthe objective multilayer film.

[0073] The conditions for the above exposure to radiation may beappropriately set depending on the purpose thereof, such as a requiredlevel of crosslinkage. For example, it is preferred to effect theelectron beam exposure at an acceleration voltage in the range of150-500 kilo-volts to provide an absorbed dose of 10-200 kGy (kilo-gray)or effect Γ-ray exposure at a dose rate of 0.05-3 kGy/hour to provide anabsorbed dose of 10-200 kGy.

[0074] It is also possible that the inner surface or/and the outersurface of the stretch-oriented multilayer film of the present inventionare subjected to corona discharge treatment, plasma treatment or flametreatment.

[0075] The stretch-oriented multilayer film of the present invention hasa remarkably improved low-temperature impact resistance and isparticularly suitable for use as a freeze packaging material. However,the presumably-caused extreme orientation relaxation at the amorphousportion represented by the improved low-temperature impact resistance incombination with the high degree of orientation of the crystallineportion has resulted in softness and improvements in piecing strength,etc., which have not been achieved heretofore. As a result, thestretch-oriented multilayer film of the present invention is alsoextremely suitable for use as, e.g., deep drawing packaging material,vertical pillow packaging material, tray packaging lid material, andpackaging material for cold or refrigeration circulation or freezepackaging material for rib (meat), fish meat and marine products such ascrabs, for which the above-mentioned properties are particularlydesired.

EXAMPLES

[0076] Hereinbelow, the present invention will be described morespecifically based on Examples and Comparative Examples. It should benoted however that the scope of the present invention is not restrictedby such Examples. Some physical properties described herein are based onvalues measured according to the following methods.

[0077] <Physical Property Measurement Methods>

[0078] 1. Impact Strength and Energy

[0079] Measured at −10° C. according to ASTM D3763-86 by using“DROP-WEIGHT TESTER RTD-5000” (available from Rheometrics, Inc.)

[0080] More specifically, in an environment of −10° C., a sample ofstretch-oriented multilayer film cut into a square of 10 cm×10 cm isdisposed horizontally and sandwiched between a pair of clamps eachhaving a 3.8 cm-dia. circular opening with its surface layer (a)directed upwards. Onto the sample film at the opening, a plunger of 4 kgin weight and having a hemispherical tip portion of 1.27 cm in diameteris dropped at a speed of 333.33 cm/sec to measure a load applied to thedropping plunger and a displacement by a sensor, from which adisplacement-load curve is obtained. Based on the curve, a maximum loaduntil the breakage is read as an impact strength (F_(IP) (N), and anenergy absorbed by the film until the breakage is calculated to obtainan impact energy (E_(IP) (J)). Five sample films from each product filmare subjected to the above measurement, and the average values are takenas measured values.

[0081] Based on the above-measured impact energy (E_(IP) (J)) for asample having a thickness t (μm), an impact energy normalized at athickness of 50 μm (E_(IP50) (J)) is calculated according to thefollowing equation:

E _(IP50)(J)=E _(IP)(J)×(50/t).

[0082] 2. Piercing Strength

[0083] In an environment of 23° C. and 50% RH, a piercing pin having ahemispherical tip having a radius of curvature of 0.5 mm attached to atensile tester (“TENSILON RTM-100”, available from Orientec K. K.) iscaused to pierce a sample multilayer film from its surface layer (a)side at a speed of 50 mm/min, thereby measuring a maximum value of forceapplied to the film until the breakage thereof as a piercing strength(F_(p) (N)).

[0084] 3. Hot-water Shrinkability

[0085] A sample film on which marks are indicated at a distancetherebetween of 10 cm in each of a machine direction (MD) and atransverse direction (TD) perpendicular to the machine direction, isdipped for 10 sec. in hot water adjusted at 90° C. and then taken outtherefrom, followed by immediate quenching within water at roomtemperature. Thereafter, the distance between the marks is measured anda decrease in distance is indicated in percentage of the originaldistance 10 cm. Five sample films from each product film are subjectedto the above measurement, and the average value of percentage decreaseis indicated in each of the MD and TD.

[0086] 4. Dry heat-Shrinkability

[0087] A 3 mm-thick corrugated board is placed on a rack, and a Geeroven (“Model MOG-600”, available from K. K. Robert) is placed thereonand heated to a prescribed temperature. Into the oven, a sample film onwhich marks are indicated at a distance therebetween of 10 cm in each ofMD and TD is placed. In this instance, the door of the oven isimmediately closed after the placement of the sample film so that thedoor opening period is restricted to be within 3 minutes. After the doorclosure, the sample film is left standing for 30 sec in the Geer ovenand then taken out for natural cooling. Thereafter, the distance betweenthe marks on the sample film is measured, and a decrease in distance isindicated in percentage of the original distance 10 cm. Five samplefilms from each product film are subjected to the above measurement, andthe average value of percentage decrease is indicated in each of the MDand TD.

[0088] <Film Production Examples>

[0089] Next, Examples and Comparative Examples for production ofstretch-oriented multilayer films are described. Resins used in thefollowing productions examples are inclusively shown in Table 1 togetherwith their abbreviations.

Example 1

[0090] By using an apparatus having an arrangement as roughly shown inFIG. 1, a tubular laminate product (parison) having a laminar structurefrom the outer to the inner layers of PET (3)/mod-VL (2)/NY-1(13)/EVOH(4)/mod-VL (2)/LLDPE (31) with thickness ratios of respective layersindicated in the parentheses was co-extruded by extruding the respectiveresins through a plurality of extruders 1 (only one being shown)respectively and introducing the melted resins to an annular die 2 tomelt-bond the respective layers in the above-described order. The moltenparison 3 extruded out of the die 2 was quenched to 10-18° C. by a waterbath 4 to form a flat tubular product 3 a. Then, the flat tubularproduct 3 a was passed through a warm water bath 6 at 92° C. and formedinto a bubble-shaped tubular film 3 c, which was then biaxiallystretched at ratios of 3.4 times in MD and 3.4 times in TD by theinflation process while being cooled with cooling air at 15-20° C. froman air ring 9. Then, the biaxially stretched film 3 d was guided into a2 meter-long heat-treating tube 12 to form a bubble-shaped tubular film3 e, which was then heat-treated for 2 sec. with steam at 90° C. blownout of steam blowing ports 13, while being allowed to relax by 20% in MDdirection and by 20% in TD direction, thereby providing a biaxiallystretched film (stretch-oriented multilayer film) 3 f. The thus-obtainedmultilayer film exhibited a lay-flat width of 490 mm and a thickness of55 μm.

[0091] The laminate structure, film production (stretching-relaxation)conditions, physical properties and packaging performances of thethus-obtained multilayer film are inclusively shown in Tables 2 to 5together with those of multilayer films obtained in other Examples andComparative Examples.

Examples 2-15 and Comparative Examples 1, 2, 4, 5 and 7

[0092] Various multilayer films were prepared in similar manners as inExample 1 except that the laminar structures and film production(stretching-relaxation) conditions were respectively changed as shown inTables 2 to 4.

Comparative Example 3

[0093] A commercially available 15 μm-thick stretched film of nylon 6(O-Ny 6) and a commercially available 60 μm-thick unstretched film ofethylene-vinyl acetate copolymer (EVA) was applied to each other to forma composite film.

Comparative Example 6

[0094] Respective resins were melt-extruded from a plurality ofextruders and the melt-extruded resins were introduced into a T-die tobe melt-bonded so as to provide a laminar structure and thickness ratiosas shown in Table 4, thereby forming a co-extruded unstretched film.

[0095] Each of the multilayer films obtained in the above Examples andComparative Examples was subjected to the above-mentioned measurement ofphysical properties and performance evaluation tests describedhereinafter. The results are inclusively shown in Tables 2 to 5described hereinafter.

[0096] <Performance Evaluation Tests>

[0097] 1. Hot Fill Performance

[0098] A 200 mm-wide and 400 mm-long pouch was formed from a samplefilm, and a hot water of ca. 70° C. was poured thereinto to evaluate thehot fill performance according to the following standard:

[0099] A: A shrinkage after the pouring of hot water was at most 5%,thus showing adaptability to hot filling.

[0100] C: A shrinkage after the pouring of hot water exceeded 5%, thusshowing non-adaptability to hot filling.

[0101] 2. Anti-pinhole Property

[0102] Each product film of Examples and Comparative Examples wereformed into bags of 220 mm-width and 450 mm-length (inner sizes). Intoeach bag, a frozen tuna cut piece (with skin) of 800 g cooled to −50° C.was vacuum-packaged in an environment of ca. 15° C. to obtain a packagedproduct. Twenty packaged product samples were prepared from each productfilm and packed into two foam styrol boxes (size: 390 mm-L×330 mm-W×260mm-H) together with dry ice so that each box contained 10 packagedproduct samples together with dry ice. The boxes were transported on antruck from Shizuoka prefecture to Ibaraki prefecture (over a distance ofca. 300 kilometers). Then, the packaged product samples were checkedwith respect the presence or absence of pinholes, and the percentage ofbroken bag was calculated by [(number of bags with pinholes)/20]×100,whereby each product film was evaluated based on the broken bagpercentage according to the following standard.

[0103] A: No bags with pinholes (Broken bag percentage=0%)

[0104] B: Broken bag percentage ≦5%

[0105] C: Broken bag percentage exceeded 5%, thus showing a problem inpractical utility.

[0106] The occurrence of pinholes showed a clear correlation with thelow-temperature impact resistance and the piercing strength, and theproduct films of Examples showed remarkably better anti-pinhole propertythan the laminate film including a substantially identical thickness ofnylon layer

[0107] (Comparative Example 3).

[0108] 3. Vertical pillow packaging performance

[0109] Each sample film was evaluated with respect to a vertical pillowpackaging performance by using a vertical pillow packaging machine(“ONPACK 207 SG”, made by Orihiro K. K.) used for packing a liquid orpowdery product by intermittent packaging operation. For the test, 1 kgof water at room temperature was packed under the conditions of alongitudinal seal temperature of 160° C., a transverse seal temperatureof 170° C. and a film cut length of 280 mm.

[0110] The packaging performance was evaluated with respect tocoming-off of the film and sticky adhesion onto a seal bar according tothe following standard.

[0111] (Film Coming-off)

[0112] A: Continuous packaging was possible

[0113] C: The film came off the seal bar due to shrinkage, thus failingto achieve continuous packaging.

[0114] (Sticky Adhesion onto a Seal Bar)

[0115] A: Continuous packaging was possible without causing stickyadhesion of the film onto the seal bar.

[0116] C: Continuous packaging was impossible due to sticky adhesion ofthe film during the packaging operation.

[0117] 4. Deep Drawing Performance (Base Sheet)

[0118] A deep drawing packaging test was performed by using each samplefilm as a base sheet together with the film of Example 3 as a lidmaterial by means of a deep drawing packaging machine (“FV-603”, made byOhmori Kikai Kogyo K. K.), wherein each sample film as a base sheet wasdeeply drawn at 100° C. by using a disk-shaped mold of 98 mm in diameterand 30 mm in depth (except that a mold of 60 mm in depth was used inExample 14). The evaluation was performed with respect to the followingitems.

[0119] (1) Formability

[0120] A: Formable without breakage

[0121] C: Deep drawing was impossible due to breakage

[0122] (2) Hot-water Shrinkability and Piercing Strength at the DrawnCorner.

[0123] A hot-water shrinkability and a piercing strength (N) weremeasured with respect to the drawn corner of a formed base sheet sample.

[0124] (3) Wrinkles after Boiling

[0125] A packaged product of indefinitely shaped roasted pig meat wasdipped in hot water at 90° C. for 10 sec., and then the presence orabsence of wrinkles on the package surface was checked.

[0126] A: The packaged product surface was free from wrinkles andexhibited a beautiful appearance.

[0127] C: The packaged product surface showed wrinkles, thus loweringthe commercial value.

[0128] (4) Abuse Test

[0129] Crylichical rubber sheets (weight=ca. 60 g/sheet) each having athickness of 5 mm and a diameter of 98 mm were packed to providepackaged products each containing 5 rubber sheets. The packaged productsamples were placed in a hexagonal tube box (which was supportedrotatably about a shaft extending horizontally to pierce the centers oftwo mutually parallel hexagonal sides of the box) and subjected to a6-angle rotation test (abuse test) for 10 min. in an environment of 5°C. The packaged product samples after the test were subjected tomeasurement of pinhole percentage.

[0130] (5) Rib (Meat) Packaging Test

[0131] Beef rib retaining a sharp-cut back bone was vacuum-packaged withsample film bags to form 20 package product samples, and the packagedproduct samples were dipped for 1 sec. in hot water at 90° C. to beshrinked. Thereafter, the packaged products were subjected to acirculation test at 5° C. and then checked with respect to theoccurrence of pinholes. The evaluation was performed according to thefollowing standard.

[0132] A: The pinhole percentage was 5% or below.

[0133] C: The pinhole percentage substantially exceeded 5%. TABLE 1Component Resins Crystal melting Abbreviation Resin Maker (Trade name)point (° C.) Remarks ** Ny-1 nylon 6-66 copolymer Mitsubishi EngineeringPlastic 195 η_(rel) = 4.5  (wt. ratio = 80:20) K.K. (NOVAMID 2430A1)Ny-2 nylon 6-69 copolymer EMS Co. 134 (GRILON BM13SBG) Ny-3 nylon 6I-6Tcopolymer EMS Co. — (GRIVORY G21) PET ethyleneterephthalate-isophthalate Kanebo KK 228 η_(int) = 0.80 copolymer *1(BELPET IFG-8L) EVOH saponified ethylene-vinyl acetate copolymer KurarayK.K. 160 MFR = 6.5 g/10 min. (ethylene content = 48 mol %) (EVALEPG156B) VLDPE ethylene-hexene copolymer Sumitomo Kagaku K.K. 119 MFR =3.0 g/10 min. (d = 0.908 g/cm³) (SMIKASEN C53009) LLDPE ethylene-octenecopolymer Idemitsu Sekiyu Kagaku K.K. 122 MFR = 2.0 g/10 min. (d = 0.916g/cm³) (MORETEC 0238CN) SVL ethylene-octene copolymer *2 Dow ChemicalCo. 100 MFR = 1.0 g/10 min. (d = 0.902 g/cm³) (AFFINITY PL1880) mod-VLmodified very low density polyethylene *3 Mitsui Kagaku K.K. — MFR = 2.7g/10 min. (ADMER SF730)

[0134] TABLE 2 Freeze-packaging & Vertical pillow packaging (1) Ex. &Comp. Ex. Ex.1 Ex.2 Ex.3 Ex.4 Film structure 1st thickness (μm) PET 3PET 2 PET 2 PET 3 2nd mod-VL 2 mod-VL 2 mod-VL 2 mod-VL 1.5 3rd Ny-1 13Ny-1 12 Ny-1 14 Ny-1 12 4th EVOH 4 EVOH 4 EVOR 9 EVOH 4 5th mod-VL 2mod-VL 2 mod-VL 2 mod-VL 1.5 6th LLDPE 31 LLDPE 32 LLDPE 35 LLDPE 18total (μm) 55 54 64 40 Stretch ratio MD 3.4 3 3 3 TD 3.4 3 3 3.2Relaxation heating Temp. (° C.) 90 90 90 90 Percentage (%) MD 20 25 1520 TD 20 25 27 20 (Film properties) Piercing strength (N) 28 23 27 23Impact strength (N) 307 214 291 241 Impact energy (J) measured 2.7 2.02.2 2.2 at 50 μm 2.5 1.8 1.7 2.8 Hot water shrink (%) MD 11 12 10 8 90°C. TD 15 8 8 12 Dry shrink (%) MD 10 7 6 5 120° C. TD 14 5 4 6 (Freezepackaging) Hot fill A A A A Anti-pinhole A B A A Broken bag percentage0% 5% 0% — (Pillow packaging) Film coming-off — — — — Sticking to sealbar — — — — Ex. & Comp. Ex. Ex.5 Ex.6 Ex.7 Ex.8 Film structure 1stthickness (μm) PET 2 LLDPE 5 PET 3 PET 3 2nd mod-VL 2 mod-VL 1.5 mod-VL2 mod-VL 2 3rd Ny-1 12 Ny-1 14 Ny-1 14 Ny-1 16 4th EVOH 4 EVOH 4 EVOH 4EVOH 4 5th mod-VL 2 mod-VL 1.5 mod-VL 2 mod-VL 2 6th LLDPE 30 SVL 20LLDPE 34 VLDPE 40 total (μm) 52 46 59 67 Stretch ratio MD 3.2 3.2 3.23.2 TD 3.2 3.2 3.2 3.2 Relaxation heating Temp. (° C.) 90 90 95 90Percentage (%) MD 20 20 28 25 TD 20 20 35 30 (Film properties) Piercingstrength (N) 22 18 18 19 Impact strength (N) 257 193 214 228 Impactenergy (J) measured 2.3 1.8 1.9 2.3 at 50 μm 2.2 2.0 1.6 1.7 Hot watershrink (%) MD 8 6 2 6 90° C. TD 12 10 3 9 Dry shrink (%) MD 5 6 4 3 120°C. TD 7 8 4 4 (Freeze packaging) Hot fill — A A A Anti-pinhole — B A ABroken bag percentage — 5% 0% 0% (Pillow packaging) Film coming-off A —A A Sticking to seal bar A — A A

[0135] TABLE 3 Freeze-packaging & Vertical pillow packaging (2) Ex. &Comp. Ex. Ex. 9 Ex. 10 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Comp. Ex. 4Comp. Ex. 5 Film structure 1st thickness PET 2 PET 2 PET 1.5 LLDPE 3O-Ny6 15 PET 4 Ny-1 10 (μm) 2nd mod-VL 2 mod-VL 2 mod-VL 1.5 mod-VL 1.5EVA 60 mod-VL 1.5 mod-VL 2 3rd Ny-1 14 Ny-1 11 Ny-1 8 Ny-1 12 Ny-1 20EVOH 3 4th EVOH 4 EVOH 4 EVON 5 EVON 4 EVOH 3 TPU 3 5th mod-VL 2 mod-VL2 mod-VL 1.5 mod-VL 1.5 mod-VL 1.5 mod-VL 2 6th LLDPE 44 LLDPE 40 LLDPE21 LLDPE 20 VLDPE 50 VLDPE 20 total (μm) 68 61 38.5 42 75 80 42 Stretchratio 3.2 3.2 3.1 3 2.4 2.9 MD TD 3.2 3.2 3.2 3.2 2.8 2.8 Relaxationheating Temp. (° C.) 90 90 70 90 70 none Percentage (%) 25 25 10 15 10none MD TD 25 25 10 15 10 none (Film properties) Piercing 20 19 17 17 1422 15 strength (N) Impact 239 218 160 160 143 225 176 strength (N)Impact energy 2.2 1.9 0.9 1.0 0.8 1.5 1.2 (J) measured at 50 μm 1.6 1.61.2 1.2 0.5 0.9 1.4 Hot water- 9 7 28 10 0 17 34 shrink (%) MD 90° C. TD13 13 30 8 0 24 30 Dry shrink (%) 4 4 21 6 0 — — MD 120° C. TD 7 7 23 60 — — (Freeze packaging) Hot fill — — C A A — — Anti-pinhole — — C C C —— Broken bag — — 30% 30% 60% — — percentage (Pillow packaging) Filmcoming- A A C A — — — off Sticking to seal A A A C — — — bar

[0136] TABLE 4 Deep drawing Ex. & Comp. Ex. Ex. 11 Ex. 12 Ex. 13 Comp.Ex. 1 Comp. Ex. 6 Comp. Ex. 7 Ex. 14 Film structure 1st thickness PET 2PET 2 PET 2 PET 1.5 Ny-1 22 LLDPE 3 PET 3 (μm) 2nd mod-VL 2 mod-VL 2mod-VL 2 mod-VL 1.5 mod-VL 10 mod-VL 1.5 mod-VL 2 3rd Ny-1 13 Ny-1 15Ny-1 + 12 Ny-1 8 EVOH 14 Ny-1 12 Ny-1 16 Ny-2 = 50 + 50 wt % 4th EVOH 7EVOH 9 EVON 4 EVON 5 Ny-1 44 EVOH 4 EVOH 8 5th mod-VL 2 mod-VL 2 mod-VL1.8 mod-VL 1.5 mod-VL 10 mod-VL 1.5 mod-VL 2 6th LLDPE 30 LLDPE 35 LLDPE30 LLDPE 21 LLDPE 101 LLDPE 18 LLDPE 40 total (μm) 56 65 52 38.5 200 4071 Stretch ratio 2.9 3 3.6 3.1 none 3 3.2 MD TD 3 3 3.2 3.2 none 3.2 3.2Relaxation heating Temp. (° C.) 90 90 90 70 none 90 95 Percentage (%) 2015 20 10 none 15 30 MD TD 20 27 20 10 none 15 32 (Film properties)Piercing 21 27 20 17 17 20 strength (N) Impact 238 291 214 160 160 250strength (N) Impact energy 2.0 2.2 1.8 0.9 1.0 2.3 (J) measured at 50 μm1.8 1.7 1.7 1.2 1.3 1.6 Hot water- 7 10 12 28 0 10 3 shrink (%) MD 90°C. TD 13 8 15 30 0 8 5 (Deep drawing (base sheet)) Formability A A A C AA A Surface gloss A A A A C A Maximum 24 24 27 bro- 29 50 drawn depthken Piercing 1.8 2.1 1.6 1.5 — strength (N) Hot water 22 19 30 5 —shrink Wrinkles A A A C A Abuse test 0% 0% 0% 0% 0%

[0137] TABLE 5 Rib (meat) packaging Ex. & Comp. Ex. Ex. 15 Filmstructure 1st thickness (μm) PET 4 2nd mod − VL 2.5 3rd Ny − 1 + 34 NY −3 = 80 + 20 wt % 4th EVOH 5 5th mod − VL 2.5 6th VLDPE 80 total (μm) 128Stretch ratio MD 2.9 TD 3.3 Relaxation heating Temp. (° C.) 75Percentage (%) MD 25 TD 25 (Film properties) Piercing strength (N) 38Impact strength (N) 440 Impact energy (J) measured 5.9 at 50 μm 2.3 Hotwater shrink (%) MD 20 90° C. TD 24 Dry shrink (%) MD — 120° C. TD —(Rib packaging) Pinhole resistance A

[0138] [INDUSTRIAL APPLICABILITY]

[0139] As describe above, according to the present invention, it hasbecome possible to produce a stretch-oriented multilayer film includinga polyamide resin layer as a principal intermediate layer and having aremarkably improved low-temperature impact resistance while retainingnecessary strength through a combination of a high degree of stretchingand a high degree of relaxation heat treatment at degrees which have notbeen exercised heretofore. Accompanying the improvement inlow-temperature impact resistance, the stretch-oriented multilayer filmis provide with improvements in piercing strength, anti-pinholeproperty, etc., thus being suitably used not only as a freeze packagingmaterial but also as a deep drawing packaging material, a verticalpillow packaging material, and also a tray packaging lid material.

1. A stretch-oriented multilayer film, comprising at least three layersincluding a surface layer (a) comprising a thermoplastic resin, anintermediate layer (b) comprising a polyamide resin and a surface layer(c) comprising a sealable resin, said multilayer film exhibiting animpact energy of at least 1.5 Joule at a conversion thickness of 50 μmat −10° C.
 2. A multilayer film according to claim 1, exhibiting anactual impact energy at −10° C. of at least 1.6 Joule.
 3. A multilayerfilm according to claim 1 or 2, wherein the surface layer (a) has alarge heat resistance than the surface layer (c).
 4. A multilayer filmaccording to any of claims 1 to 3, wherein the intermediate layer (b)has a larger thickness than the surface layer (a).
 5. A multilayer filmaccording to any of claims 1 to 4, wherein the surface layer (a)comprises a stretch-oriented polyester resin.
 6. A multilayer filmaccording to any of claims 1 to 5, having a heat-shrinkability.
 7. Amultilayer film according to claim 6, having a hot-water shrinkabilityat 90° C. of below 20%.
 8. A multilayer film according to claim 6,having a hot-water shrinkability at 90° C. of below 15%.
 9. Afreeze-packaging material, comprising a stretch-oriented multilayer filmaccording to any one of claims 1 to
 8. 10. A vertical pillow-packagingmaterial, comprising a stretch-oriented multilayer film according to anyone of claims 1 to
 8. 11. A deep drawing-packaging material, comprisinga stretch-oriented multilayer film according to any one of claims 1 to8.
 12. A tray-packaging lid material, comprising a stretch-orientedmultilayer film according to any one of claims 1 to
 8. 13. A process forproducing a stretch-oriented multilayer film, comprising the steps of:co-extruding at least three species of melted thermoplastic resins toform a tubular product comprising at least three layers including anouter surface layer (a) comprising a thermoplastic resin other thanpolyamide resin, an intermediate layer (b) comprising a polyamide resinand an inner surface layer (c) comprising a sealable resin, cooling withwater the tubular product to a temperature below a lowest one of themelting points of the thermoplastic resin, the polyamide resin and thesealable resin constituting the layers (a), (b) and (c), re-heating thetubular product to a temperature which is at most the lowest one of themelting points of the thermoplastic resin, the polyamide resin and thesealable resin constituting the layers (a), (b) and (c), verticallypulling the tubular product while introducing a fluid into the tubularproduct to stretch the tubular product in the vertical direction and thecircumferential direction, thereby providing a biaxially stretchedtubular film, folding the tubular film, again introducing a fluid intothe folded tubular film to form a tubular film, heat-treating thetubular film from its outer surface layer (a) with steam or warm wateruntil a relaxation ratio reaches at least 20% in at least one of thevertical direction and the circumferential direction, and cooling theheat-treated tubular film to provide a stretch-oriented multilayer filmexhibiting an impact energy of at least 1.5 Joule at a conversionthickness of 50 μm at −10° C.
 14. A process according to claim 13wherein the biaxially stretched tubular film is formed by stretching thetubular product at ratios of at least 2.9 times in a vertical directionand at least 3 times in a circumferential direction while verticallypulling the tubular product.
 15. A process according to claim 13 or 14,wherein the tubular film is heat-treated for the relaxation with steamor warm water at 75-95° C.